In a high intensity discharge (HID) lamp, a medium to high pressure ionizable gas, such as mercury or sodium vapor, emits visible radiation upon excitation typically caused by passage of current through the gas. One class of HID lamps comprises electrodeless lamps which generate an arc discharge by generating a solenoidal electric field in a high-pressure gaseous lamp fill. In particular, the lamp fill, or discharge plasma, is excited by radio frequency (RF) current in an excitation coil surrounding an arc tube. The arc tube and excitation coil assembly acts essentially as a transformer which couples RF energy to the plasma. That is, the excitation coil acts as a primary coil, and the plasma functions as a single-turn secondary. RF current in the excitation coil produces a time-varying magnetic field, in turn creating an electric field in the plasma which closes completely upon itself, i.e., a solenoidal electric field Current flows as a result of this electric field, resulting in a toroidal arc discharge in the arc tube.
A high frequency ballast is required to convert ac power line frequencies to the radio frequencies necessary to induce an arc discharge in the fill within the arc tube of an electrodeless HID lamp. An exemplary ballast comprises a Class-D power amplifier, such as that described in commonly assigned U.S. Pat. No. 5,047,692 of J. C. Borowiec and S. A. El-Hamamsy, issued Sep. 10, 1991. The power switching devices of the ballast require effective heat sinking in order to protect the junctions thereof from excessive temperature rises which would otherwise increase the device resistance, and hence decrease efficiency. Furthermore, operating the power switching devices at excessively high temperatures reduces the useful life thereof.
The excitation coil of an HID lamp requires effectual heat sinking in order to maximize efficiency of the system. In particular, the excitation coil of an electrodeless HID lamp surrounds the arc tube. As a result, the coefficient of electromagnetic coupling between the coil and the solenoidal discharge is relatively low, typically in the range from about 0.2 to 0.4. Therefore, in order to produce a predetermined discharge current in the arc tube, an even larger current is required in the coil. The relatively large coil current results in resistive losses in the coil that can have a significant deleterious effect on efficiency of the overall HID lamp system. Moreover, as the temperature of the excitation coil increases, coil resistance increases. Hence, to increase efficiency of an electrodeless HID lamp system, heat resulting from coil resistive losses and from convection and radiation from the hot arc tube to the coil must be removed.
In addition to the heat sinking requirements of the excitation coil and the ballast, a low-inductance electrical connection from the coil to the ballast is required in order to maximize efficiency. Furthermore, an impedance matching network is required to match the coil impedance to the ballast. Accordingly, it is desirable to provide a ballast and excitation coil configuration which is suitable for use in an electrodeless HID lamp fixture, which configuration optimizes both the heat-sinking and electrical requirements of the system and hence maximizes efficiency of the lamp and increases the useful life thereof.